...The Valeyard.
In all seriousness, though, I could see both One and Six, but in different ways — One could become that person who just cares about research and nothing and nobody else, ruining (accidentally or deliberately) lots of people’s lives in the process. On the other hand, I can see Six as something more like the Time Lord Victorious.
One also basically kidnapped his first companions and risked the whole team’s lives by lying about something just to find something out, and Six was pretty full of himself and nearly strangled Peri,
I feel most people are going to go for Six, though, so I’m going with One.